The detection and classification of a live being, human or other, has a variety of useful applications in the field of security (intruder detection), safety (trapped occupant detection) and personal health and medical applications. A variety of systems have been described, developed or even provided for such purposes, for building rooms, vehicles and other defined volumes. A technology used in many of these systems is ultrasound: that is, acoustic wave signals at frequencies above the normal range of human hearing. Prior art systems are described that transmit ultrasonic signals in a pulse mode and analyze returning, reflected signals using a “time of flight concept;” while other such systems employ continuous transmission and analyze the returning, reflected signals for Doppler shift. Many of the systems are successful in certain applications and within certain limits, in detecting a moving occupant and generating an alert that is to be interpreted as a detection of a living occupant. But the systems are hampered by their inability to deal with problems in acoustic impedance of transmission media and reflecting occupants and wave interference in the received signals between transmitted and reflected acoustic waves. They generally tend to be limited to the detection of gross body movements, and are not sufficiently sensitive to reliably detect the small movements characteristic of respiration that involve only very small body movements.